1. Field of the Invention
This invention relates to the division of an input optical beam into an array of multiple output beams, or the reverse process in which multiple input beams are combined into a single output beam, and more particularly to such a beam reconfiguration that is accomplished by controlling an electrode array in a liquid crystal cell.
2. Description of the Related Art
With the emergence of fiber optic communications and the extensive use of optical links in general, there is a need for systems that can manipulate optical signals with a greater degree of flexibility and adaptability than has previously been available (the term "optical" as used herein includes infrared, ultraviolet and other regions of the electromagnetic spectrum, and is not limited to visible light). One optical processing function that is of particular interest for the present invention is a star coupler, in which light from one input beam is distributed to multiple output beams that diverge from each other, or multiple converging input beams are combined into a single output beam. Optical beam division and combination have previously been performed by stationary or mobile mirrors with complete or partial reflectivity, refractive or holographic lenses, prisms and fixed grating rulings. For example, periodic mechanical structures in the form of grooves, multi-pinhole masks and faults in periodic structures are employed for beam splitting in Dammann et al., "Coherent optical generation and inspection of two-dimensional periodic structures", Optica Acta, Vol. 24, No. 4, 1977, pages 505-515, while a binary phase grating with arbitrarily shaped grooves to implement a star coupler is described in Killat et al., "Binary Phase Gratings for Star Couplers with High Splitting Ratio", Fiber and Integrated Optics, Vol. 4, No. 2, 1982, pages 159-167. Such opto-mechanical devices are bulky and sluggish, and are designed to operate on a fixed number of optical beams at fixed prescribed angles and spacings. Major structural modifications are required to reconfigure the systems to handle beams with different angles of incidence and transmission.
Dynamic control over beam processing has been achieved with the use of multiple electrode systems in a liquid crystal cell. However, these systems have been limited to deflecting a single input beam into a single beam output. While the angle of deflection can be dynamically varied, an incident beam is not divided into multiple outputs, nor are multiple converging inputs combined into a single output. Such systems are described in U.S. Pat. No. 4,937,539 to Grinberg et al., assigned to Hughes Aircraft Company, the assignee of the present invention, and also in U.S. Pat. No. 4,639,091 to Huignard et al. In these systems an optical phase grating is established in the liquid crystal cell by dividing the cell into multi-electrode periods, and applying similar voltage gradients to the electrodes within each period. Thus, the voltage on the electrode at one end of each period will be an a minimum level, the voltage on the electrode at the opposite end of each period will be at a maximum level, and the voltages on the intermediate electrodes progressively ramp up between the minimum and maximum values. This produces an optical grating that allows the input beam to be steered in accordance with the slope of the voltage ramp across each period. However, these patents are limited to beam deflection, and do not include beam splitting or combining.
If an attempt were made to divide the aperture of the above patents into a number of subapertures, with each sub-aperture deflecting its portion of the incident beam in a different direction from the other subapertures, the beam-width of each output beam would be determined by the size of its associated subaperture. A disadvantage of this approach is that, as the number of apertures increases, the size of each subaperture decreases and the beamwidth of the output beams increases by diffraction.